The present invention relates generally to DC electrical systems, and more particularly, to a method and apparatus for detecting a loose electrical connection in a photovoltaic (PV) system.
The US and other countries have been experiencing record numbers of PV installations in recent years. In one recent year, for instance, the US experienced 339 MW of grid-connected PV during the first 6 months of the year, which represents a 55% increase over the 435 MW that was installed in the entire previous year. Not only has the number of systems increased dramatically in recent years, but the number of large scale systems has increased as well.
Generally, as known in the art, a PV system includes individual solar modules that are connected in series to form a string of, typically, 8-12 modules. A group of strings are connected in parallel in a combiner box, which typically includes a fuse for each positive string wire, and the fuse(s) feed a positive bus bar. Negative wires are also collected within the combiner box to form a negative bus. Conductors sized to handle the combined current and voltage produced at the combiner boxes carry DC power to a master combiner (which may also be regarded as an array combiner or a re-combiner), where combiner box outputs are combined in parallel. Output from one or more master combiners travels through large conductors to a central inverter, and DC power from the master combiner is output as AC power from the inverter. The inverter output is fed to a transformer that converts the output AC voltage to the utility's transmission voltage.
PV systems are expected to be highly robust and reliable for at least twenty years of operation. However, like many high voltage electrical systems, PV systems are susceptible to failure due to, among other things, loose connections resulting in overheating and arcing that can occur in the system. Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of electrodes. An arc fault is an unintentional arcing condition in an electrical circuit and can be caused by, for instance, worn conductor insulation, exposed ends between broken conductors, faulty electrical connections, or loose connections where conducting elements are in close proximity to each other, as examples. Depending on the current, the plasma formed during an arcing fault can reach temperatures in excess of 5000 degree C. in a very localized area. This heating can be sufficient to melt surrounding components that are made of plastic or metal, such as fuse holder, parts of disconnect switches, and even the combiner box enclosure itself. This can lead to injury, equipment and property damage, and fires due to ignition of building or PV materials, threatening the loss of building contents and occupant safety.
PV systems are at risk of developing a fault due to the very large number of connections in the system. Thousands of connections can exist in a PV system, giving thousands of opportunities for poor connections. A large PV system can have over one hundred combiner boxes, as an example. Thus, there can be thousands or even many thousands of opportunities for faults to occur. Bus bar connections are typically bolted together, and there can be any number of these bolted structures within each combiner box. Within a combiner box, field terminated strings and bus feed wiring particularly have a high potential for developing loose connections, and bus bars and associated termination hardware also have the potential to become loose through electrical and thermal cycling. The risk of developing a fault is even higher for PV strings where the power propagates down the PV wiring, through PV connectors, as well as through PV module ribbon and cell interconnections.
More so, PV systems are particularly at risk because of damage from sun, wind and weather that can occur over system working life and from the conditions that occur where PV systems are typically installed. That is, the relatively harsh conditions on building roofs, in open fields, etc. . . . can lead to physical damage and accelerated aging of the PV system. Exposure to wind, harsh winter cold and extreme summer heat can weaken connections anywhere throughout the system, causing loose connections. And, because of the dramatic growth in the number of deployed PV systems in recent years, the risk of fire and other damage has only increased. In fact, PV fires have been reported in recent years that have been traced back to component overheating and arcing, particularly caused by loose connections.
Safety, protection, and service requirements for DC components and circuits in PV systems have lagged the code requirements, standards, and experience established for AC systems. Protection systems for AC power distribution have progressed from short circuit (fuses and circuit breakers) and overcurrent (protective relays) to ground fault and arc fault protection. One known AC loose connection detection/protection system, applicable to switchgear, switchboard, and motor control centers, is based on passive acoustical sensing with a piezo sensor and an Event Time Correlation (ETC) algorithm. That is, this known AC protection system detects acoustic noise generated within the system itself and, with the ETC algorithm, can be used to pinpoint the source of loose connections.
In other words, because of the cyclical nature of AC power and its passing through zero points, a loose connection within the AC system can manifest itself as a vibration between the loose components. The vibration in turn is detectable as an acoustic noise with piezo sensors positioned throughout the system. Based on the time of travel to one or more piezo sensors the locations of the source can be determined. As such, a loose connection within an AC system can be detected, often before the loose connection proceeds to the point where overheating or an arc may be formed.
DC power systems on the other hand, such as a PV system, do not inherently generate acoustic signals in loose connections. Generally the components remain in a static position if loose, unlike in a typical AC circuit, and will remain so until an arc forms. As such, DC power systems may not have loose connections detected therein, using known systems, until it is too late and the arc has formed, or worse yet when the arc has progressed to the point of becoming a fire hazard.
Further, a fault or loose connection in a PV system may not be detectable during darkness because the risk of arcing has abated when the system is not under power. Thus, in order to proactively detect faults or loose connections, the PV system must be monitored during daylight hours and when the PV modules are generating power.
As such, it would therefore be desirable to have a system and method capable of detecting loose connections in a DC electrical power circuit and, more particularly, in a PV system, that overcomes the aforementioned drawbacks.